Here are some descriptions of a foul smell that has stunk its way across a huge stretch of Southern California.

PAT STEVENS: Rotting fish, sewage, you know.

JOYCE THATCHER: It smells exactly like somebody's septic system overflowed.

SEAN NEALON: Like an old banana under the seat for, like, a week, and it just turns all black and gooey and, like, something's rotting.

BLOCK: That was Pat Stevens in Banning, California, along with Joyce Thatcher and Sean Nealon in Riverside. The smell appears to have wafted from the Salton Sea, more than 150 miles north and west, up to the San Fernando Valley. So what's causing the big stink?

Jack Crayon is an environmental scientist for California's Department of Fish and Game. And he joins me now from Bermuda Dunes.

Jack, how would you describe the smell?

JACK CRAYON: I'd say a rotten egg smell. You know, one of the major components of this event, and we have several each year, is hydrogen sulfide gas that's being brought up from the bottom of the sea. And that's what you smell when you smell a rotten egg. It's a gas that's created by bacteria in the absence of oxygen.

BLOCK: You say this is not an unusual thing to happened, but the smell seems to be much stronger this year and certainly has gone much, much further.

CRAYON: That's largely a function of weather patterns. When you live down close to the sea - and I'm only about 15 miles away - you'll smell this. The locals are familiar with this. But the prevailing weather patterns usually bring winds from the north, from the L.A. area. We had a couple of storms recently where there were strong winds blowing up from the south. And that's more than likely the mechanism that's bringing this up to people who probably, though, couldn't even point the direction of the Salton Sea.

BLOCK: And what exactly is going on in the Salton Sea that would be causing this smell?

CRAYON: There's too many nutrients, too much of a good thing. And basically it's an ecosystem and that's having indigestion.

At the bottom of the sea there is a lot of organisms - whether it's an algae bloom or fish or duck poop - whatever goes down there will break down, but it's in an oxygen-free zone at the bottom of the sea and it builds up in the water. So water accumulates these gases, like hydrogen sulfide and ammonia. And when you have a wind event that will churn the waters of the sea, it brings these up and releases them. And that's what the smell is all about.

BLOCK: You know, the name Salton Sea is a little misleading because it's an inland lake, right?

CRAYON: Yeah.

BLOCK: It is salty.

: Yes.

BLOCK: And I gather that there's been a lot of concern. I was reading a warning that the system is going to experience a catastrophic event because of these excessive nutrients that you're talking about, high salt levels, and the ecosystem basically collapses under an algae bloom.

CRAYON: Yeah. I don't know about catastrophic. It's more like a continual degradation of the values of the sea. There's no outlet, so the sea is becoming increasingly more salty, increasingly more full of nutrients. Sooner or later it's not going to support fish. And that's when you would have what I'd call an ecological catastrophe.

BLOCK: Has there been a big fish die-off this season?

CRAYON: Well, big's a relative term here. I don't think we've had the scale of past large ones with millions of fish. But we see a number of smaller die-offs during some moderate winds, and fish will be killed during that event.

BLOCK: Wait. The wind would precipitate the fish die-off?

CRAYON: Exactly. As the wind blows across the sea - the sea is very shallow now, you have to understand. If you shrunk the Salton Sea to the size of a football field and kept it to scale, it would only be one inch deep. It's very shallow. The winds blow across the sea, they bring up those bottom layers of water, and it releases all those gases through the water that have been collecting for weeks or even months. Those gases strip out the oxygen from the water and that's what kills the fish.

BLOCK: Jack Crayon is an environmental scientist with California's Department of Fish and Game. Mr. Crayon, thank you so much.